There are a number of common methods in wax manufacturing to get the wax into a condition or shape to package, transport and utilize and/or feed in subsequent processes, most of which likely take place at other locations. The common forms are as follows.
A) Bricks and Cakes: This form is probably the oldest process, and most elementary. Basically, the molten wax is poured into a mold of a desired shape such as circular, rectangular, etc. and let cool. The disadvantages to this method of manufacture include risk of spillage and splash on operators, long periods of storage and much space to cool, risk of contamination during pouring and generally such technique is very labor intensive. Later when using these bricks or cakes, the total mass is great and thus takes melt pots and time to re-melt the wax. Adding more bricks to an existing container partly full of liquid wax also increases risk of splash to operators. Dosing and conveying methods are not very sophisticated.
B) Prills: Prilling involves very tall towers (and thus buildings) with long insulated pipe runs, which basically utilize certain atomizing spray nozzles. The sprayed wax generally free falls in a cross-current of cooled gas a distance necessary to cool the wax droplets to a solid state. Depending on the wax and how things are adjusted, this method can yield fine particles like powder-to-small beads-to-pellets close to 2 to 3 mm in size. This process tends to work well with optimal flexibility when the waxes are of the lowest viscosity types. However, as the viscosities increase there is a challenge to get the spray concept to work, as the wax would prefer to pour as a solid stream or string. Further, since these towers are quite tall, considerable space (in height and volume) and construction work are required. Furthermore, gas cooling is not the most efficient way to process polymers. Thus, considerable energy is required to do the cooling and gas circulation functions, plus there is the large amount of construction insulation required for such large facilities and especially in locations where the ambient temperatures are quite high.
C) Slats & Chips: This form is a result of pumping and/or extruding the molten wax as continuous strand(s) onto a belt, usually a steel belt, on which the heat energy is absorbed by the belt from the wax until the wax has solidified. At the end of the belt the wax strand or strip (as the strand tends to flatten out) is feed into a cutter, thus shearing the strips into slats or chips. The disadvantage here again is inefficiency. As the strand falls upon the belt, the contact surface promptly cools/solidifies. But this forms a boundary layer that tends to insulate the remainder of the wax above it. Thus the cooling process slows down as the wax stays on the belt without any agitation or cooling surface removal/renewal. As a result the steel belts which must be of a polished finish (like a mirror) can get very long and wide to have any appreciable rate of production. These precision polished belts can be very expensive and are quite susceptible to damage, and require tremendous cooling support units. The belts can take up much floor space and in the event of rate increase (or viscosity increase), need greater and greater lengths requiring aisles to be changed in plant layout or worse. In regard to wax grades with higher and higher viscosities, they tend to require the existing belt lines to significantly slow down, so as to increase exposure to cooling time, thus resulting in considerable loss in production output. In addition, during humid seasons or in places where humidity is high, the very cold steel belts can suffer condensation build up, thus making the waxes wet (and there is usually no drying capability). To compensate, an expensive cover could be installed over and around the belt process. But this too, must be climate controlled. And without such a cover, the product is exposed to dust, insects and other contaminates, to be imbedded in the still molten waxes. There is also increased exposure to oxidation at the elevated temperature with or without the cover.
D) Pastilles: This process also utilizes a steel belt principle. As such, many of the disadvantages are the same as described above for the “slats & chips” method. One significant difference is the final wax product shape which is more like a pellet or lens shape, and the final product tends to be very uniform. Conversely, this process, which uses the principle of dropping “droplets” of wax onto the belt, is even more limited when approaching higher viscosity grades where the wax product would rather pour onto the belt instead of dripping onto the belt. Thus, this process tends to be limited to the low end viscosity range. Further “pastille” sizes can be very limited; meaning it gets less efficient and practical for the belt process to make “micro-pellet” sizes.
E) Pellets & Powder: Some prilling applications can produce “near powder” sizes or make near pellet sizes (like 2 to 3 mm) which can then be ground into powder. Some waxes are of a high enough viscosity, also having enough melt strength and a wide enough liquid to solid state temperature range to be suitably pelletized, such as by means of an underwater pelletizing process. For such wax grades, they can be sold either in pellet form (such approximately 3 mm in diameter) or can be ground into a fine powder form.
However, efforts to pelletize waxes using underwater pelletizer and centrifugal dryer equipment has attained mixed results and, in many cases, the underwater pelletizing methodology has produced unsuccessful results. Most waxes have very low liquid-to-solid temperature points, relative to the many resins, polymers, plastics, and elastomer type materials and their compounds that can be pelletized with underwater pelletizing technology. What is the basic problem for underwater pelletizing of waxes is the fact that many of these waxes go from an extremely low viscosity (much lower than normally observed in the other above mentioned polymers) to become a solid within a very narrow range of temperature, typically from about 5° C. to about 20° C. In contrast, the band or range of temperatures for many of the other polymers on which underwater pelletizing is applicable are much wider from being in a more liquid state to a more solid state. For the purposes of this application, materials having a narrow temperature range for liquid/solid state change are referred to as having a “sharp melt point”.
Materials that exhibit this sharp melt point in combination with a very low melt to solid transition temperature include most waxes. These properties can cause serious problems when attempting to pelletize waxes using underwater pelletizing equipment. The leading problem is that as the wax passes through the die plate (a metal plate with a relatively concentric circle or circles of extrusion orifices), the wax will have a tendency to freeze-off within the extrusion orifices. This is caused by the fact that underwater pelletizing utilizes a water flow across the die plate face to act as a quenching medium for the extruded strands exiting from the die orifices, and as a conveying means once the strands are cut into pellets at the orifice exit point by the rotating blades of the pelletizer cutter.
This freeze-off or freezing occurs because the water flowing across the die face is of a normally much lower temperature than the liquid or melt temperature of the wax extrudate. Thus, as the wax strand passes through the die extrusion orifice, the strand loses much of its remaining internal heat energy into the surrounding die extrusion orifice wall as it approaches the exit. And because of its sharp melt point, the wax transitions very quickly into a solid state before exiting the orifice thus creating a blockage in that orifice. As a result the back pressure forcing the wax into and through the die orifices increases and velocity through any remaining open flowing orifices also increases. Other orifices can continue to freeze and block until some sort of equilibrium velocity and back pressure are achieved, so to generally keep any remaining unblocked holes open. This situation is very unpredictable for the pelletizing process, and yields non-uniform size pellets. Thus the process is very unstable to continue.
Furthermore, increasing back pressure causes slipping within the upstream pumping equipment, which can occur easily because of the very low viscosities of the wax in the molten/liquid state. The loss of rate from the pump to the die further complicates the issue of reaching an equilibrium state, and thus further adds instability to the process. Additionally, the pumping equipment while working to create pressure and flow of the wax, while suffering slip, will add more energy into the wax, thus driving the already low viscosity, even lower, making it even more difficult to establish a stable running system with predictable pelletizing results.
Yet another problem associated with pelletizing waxes is that a common property of most waxes, unlike many of the polymers/plastics on which underwater pelletizers work well, is they have a very low “melt strength”. For the purposes of this application, the term “melt strength” is intended to define the ability of the material to stay together upon the impact of the cutting blades at high speed, to shear the polymer or wax strand as it exits the die orifice. In other words, as the strand is cooling from the influence of the process water the pellet is gaining in strength to hold itself together to be formed into a pellet.
In the case of many waxes, the melt strength is nearly non-existent, and as the liquid or semi-liquid wax strand exits the die orifice, the impact of the cutter blade trying to shear the strand into a pellet actually causes an impact explosion or shattering of the pellet into many fragments. This effect produces a wax solid geometry more like shredded coconut or like fines and/or a combination of the two.
Even if shredded coconut or fines-type particles might be acceptable, there remain the problem of how to get such wax particles separated from the water and dry. Standard centrifugal dryers that typically support the underwater pelletizer cannot be used effectively. For example, with many grades of wax where suitable/normal pellet geometry could be achieved with an underwater pelletizer, such as a 3 mm diameter cylinder, lens or sphere, these wax pellets could be brittle entering the centrifugal dryer at the colder process water temperatures, thus breaking the pellets causing waste in fines or dust. Conversely, if the water temperature is warmed to reduce breakage, the higher temperature causes the pellets to be softened and more likely to scrape off particles from the wax pellet surfaces as they pass through the dryer, thus still producing fines and dust.
Another problematic issue associated with using a centrifugal type dryer can be the pellet deformation effects inside of the dryer. This could be an issue whenever the deformation temperature of the material is below the actual temperature of the material at the time it passes into and through the centrifugal dryer. The most common problem observed is the material getting embedded onto or into the dryer rotor screens which leads to the screens getting plugged over time with the materials. This embedding and/or plugging reduces or eventually eliminates the ability of the dryer to get the material dry enough for subsequent packaging, storing or processing.